Organic Chemistry · Biomolecules: Lipids
Waxes, Fats, and Oils
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In 30 seconds
Fats, oils, and waxes are lipids — biomolecules that dissolve in nonpolar solvents but not in water. Despite their variety, nearly all share one chemical theme: they are esters of an alcohol and long-chain carboxylic acids called fatty acids.
A fat or oil (technically a triacylglycerol, or triglyceride) is the triester of glycerol with three fatty acids; the only difference between them is physical state at room temperature — fats are solid, oils are liquid. A wax Ester of a long-chain fatty acid and a long-chain alcohol Full entry →, by contrast, is the ester of a single long-chain fatty acid Long-chain carboxylic acid, e.g., CH3(CH2)16COOH Full entry → with a single long-chain monohydric (one-OH) alcohol — beeswax, apple-skin coating, and human earwax are all such esters.
The differences in physical behavior trace back to molecular structure: whether the fatty-acid chains are saturated (all single C–C bonds) or unsaturated (one or more C=C), and whether those double bonds are cis or trans. This topic builds the structural vocabulary for the rest of Chapter 27: saponification (soap), phospholipid membranes, and isoprenoid molecules.
Why this matters
Triacylglycerols are the body's chief energy store: at about 9 kcal per gram they deliver more than twice the energy density of carbohydrates or protein (about 4 kcal/g). In the kitchen, the difference between solid butter and liquid olive oil is the same chemistry behind nutrition labels' "saturated fat" and "trans fat" warnings — trans fats raised heart-disease risk and were largely removed from processed foods. Waxes waterproof plant leaves, insect exoskeletons, and feathers, and industrially they protect car finishes and fruit from moisture loss. Knowing how fatty-acid structure sets the melting point lets you predict why margarine is spreadable and why frying oil thickens with repeated use.
The college version
Core Concepts
Fatty acids: the building blocks
A fatty acid is a long-chain carboxylic acid, almost always with an even number of carbons because cells build them two carbons at a time. General formula: CH3(CH2)nCOOH. Common examples:
- Palmitic acid (16:0): CH3(CH2)14COOH — saturated
- Stearic acid (18:0): CH3(CH2)16COOH — saturated
- Oleic acid (18:1): CH3(CH2)7CH=CH(CH2)7COOH — one cis double bond
- Linoleic acid (18:2): CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH — two cis double bonds
The shorthand "18:1" means 18 carbons and 1 double bond. Linoleic acid (an ω-6) and α-linolenic acid (18:3, an ω-3) cannot be made by the body and must come from the diet — they are essential fatty acids.
Triacylglycerols: fats versus oils
A triacylglycerol forms when glycerol, HOCH2CH(OH)CH2OH, reacts with three fatty acids to give three ester linkages, releasing three waters. In shorthand, one with three stearate chains is written (C17H35COO)3C3H5 (tristearin). Natural fats are mixed triacylglycerols — different fatty acids on one glycerol backbone.
Whether the material is solid or liquid depends on how tightly chains pack. Saturated chains are straight and stack like pencils, giving strong intermolecular forces — solid fats (butter, lard). A cis double bond kinks the chain about 30°, preventing close packing and lowering the melting point — liquid oils (olive, canola). The more double bonds, the lower the melting point: coconut oil (mostly 12:0/14:0) is solid near room temperature, while soybean oil (rich in 18:2/18:3) stays liquid even refrigerated.
Hydrogenation and trans fats
hydrogenation Addition of H2 across C=C with a metal catalyst Full entry → adds H2 across C=C bonds using a metal catalyst (Ni, Pd, or Pt). Full hydrogenation gives a solid fat; partial hydrogenation (margarine, shortening) leaves some double bonds — but the catalyst also isomerizes many from cis to trans. Trans chains stay nearly straight, so trans fats pack like saturated fats, raising LDL and lowering HDL cholesterol — hence separate food-label reporting.
Waxes: esters of long-chain alcohols
A wax is an ester of a long-chain fatty acid (typically C14–C36) and a long-chain monohydric alcohol (typically C16–C36), such as myricyl palmitate, C15H31COOC30H61, the main component of beeswax. With no polar head group and very long chains, waxes are harder, more water-repellent, and less digestible than fats. Plants coat leaves to limit water loss; birds and insects use waxes for waterproofing; lanolin (wool wax) is a skin-care staple.
Rancidity: oxidation and hydrolysis
Oxidative rancidity Spoilage by oxidation or hydrolysis of fats Full entry → is auto-oxidation: oxygen attacks the allylic C–H positions next to double bonds, forming hydroperoxides that decompose into smelly aldehydes and ketones. Hydrolytic rancidity is ester hydrolysis to free fatty acids — the "soapy" taste of old butter comes from liberated butanoic acid. Refrigeration slows both, and antioxidants (vitamin E, BHA/BHT) intercept the radical chain.
How It Works / Step-by-Step Process
Classifying a lipid by structure:
- Look for ester linkages (C(=O)–O–C). No ester → not a fat, oil, or wax.
- Count the alcohol part: glycerol (3 OH groups) → triacylglycerol; a single long-chain alcohol → wax.
- Examine the fatty acid chains: any C=C? Count them (18:0, 18:1, 18:2, …).
- Decide the geometry at each double bond: cis (kinked) or trans (straight).
- Predict the physical state: long, straight, saturated chains → solid; kinked unsaturated chains → liquid.
Predicting melting point trends: more carbons → higher melting point; more cis double bonds → lower melting point; trans bonds behave like single bonds.
Common Confusions
| Common Confusion | Correct Understanding |
|---|---|
| "Fats and oils are different kinds of molecules." | They are both triacylglycerols; only the melting point (solid vs liquid at room temperature) differs. |
| "Trans fat is just another unsaturated fat." | Trans-unsaturated fats are straight-chain and pack like saturated fats; they raise LDL and lower HDL, unlike cis fats. |
| "Cholesterol is a fat you eat in fatty foods." | Cholesterol is a steroid (Topic 6), not a triacylglycerol; dietary fats are mostly triacylglycerols. |
| "All double bonds lower the melting point." | Only cis double bonds do. Trans double bonds keep chains straight, so trans fats melt higher than their cis isomers. |
| "A wax is just a hard fat." | Waxes are esters of one fatty acid and one long-chain alcohol; fats are glycerol triesters. |
| "The 18 in 18:1 means it has 18 double bonds." | It means 18 carbons; the number after the colon counts double bonds. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine beads on strings. Saturated fat beads are straight strings that lie flat and stack tightly, like uncooked spaghetti — that's solid fat. A cis double bond is a kink, like a bent pipe cleaner — the strings can't stack, so the material is a liquid oil. A wax is one very long string with no kinks, which is why wax is hard and waterproof. Your body burns fat like a furnace burns logs: slow to light, but lots of heat per log.
Worked example
Example 1: Degree of unsaturation of linoleic acid
Linoleic acid is C18H32O2. How many rings plus double bonds (DBE) does it contain?
The formula for a molecule with carbons C, hydrogens H, and oxygens O (oxygen does not affect the count) is:
DBE = 2C + 2 - H2
Substituting C = 18, H = 32:
DBE = 2(18) + 2 - 322 = 36 + 2 - 322 = 62 = 3
The carboxylic acid contributes one double bond (C=O), so the remaining 3 - 1 = 2 are the two C=C bonds — matching "18:2". DBE quickly verifies a fatty acid's structure from its formula.
Example 2: Hydrogenating 100 g of triolein
Triolein, (C17H33COO)3C3H5, is the triacylglycerol of oleic acid (18:1) with three double bonds per molecule. What mass of H2 is required to fully hydrogenate 100.0 g of triolein?
Step 1 — moles of triolein. Molar mass of C57H104O6:
M = 57(12.011) + 104(1.008) + 6(15.999) = 885.4 g/mol
n = mM = 100.0 g885.4 g/mol = 0.1129 mol
Step 2 — moles of H2. Each double bond consumes one H2; three double bonds per triolein:
nH2 = 3 × ntriolein = 3(0.1129 mol) = 0.3388 mol
Step 3 — mass of H2. MH2 = 2.016 g/mol:
mH2 = nH2 MH2 = (0.3388 mol)(2.016 g/mol) = 0.683 g
Unit check: g × (mol/g) × (mol/mol) × (g/mol) → g. Fully hydrogenating 100 g of triolein needs only about 0.68 g of H2 — a tiny mass change, but the physical change (liquid → solid) is dramatic.
Key takeaways
- Triacylglycerols = glycerol triesters of fatty acids; waxes = monoesters of a fatty acid + a long-chain alcohol.
- Fatty acid shorthand "C:D" = number of carbons : number of double bonds; natural chains are even-numbered, usually C16–C18.
- Saturated → straight chains → solid fats; cis-unsaturated → kinked chains → liquid oils; more double bonds → lower melting point.
- Essential fatty acids (linoleic, α-linolenic) must come from diet.
- Hydrogenation converts oils to solids but generates trans fats, which raise LDL and lower HDL cholesterol.
- Energy density: fats ≈ 9 kcal/g vs ≈ 4 kcal/g for carbohydrate and protein.
- Rancidity is radical auto-oxidation at allylic positions (oxidative) or ester hydrolysis (hydrolytic).
- Degree of unsaturation, DBE = (2C + 2 - H)/2, counts rings plus double bonds (oxygen contributes nothing).
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Why is butter solid at room temperature while olive oil is liquid, even though both are triacylglycerols?
Show answer
Butter's fatty acids are mostly saturated (straight chains that pack tightly); olive oil's are mostly cis-unsaturated (kinked chains that cannot pack), so its melting point is lower.
How many cis double bonds does oleic acid (18:1) have, and how does that kink affect chain packing?
Show answer
One. The cis kink bends the chain roughly 30°, disrupting van der Waals packing and lowering the melting point.
What chemical change converts a liquid oil into a solid margarine-like fat, and what unwanted isomer is produced by partial hydrogenation?
Show answer
Hydrogenation adds H2 across C=C bonds; partial hydrogenation also isomerizes remaining double bonds to trans, producing trans fats.
What is the structural difference between a wax and a fat?
Show answer
A wax is one fatty acid esterified to one long-chain alcohol; a fat is glycerol esterified to three fatty acids.
Calculate the degree of unsaturation of stearic acid, C18H36O2. Does the answer match "18:0"?
Show answer
DBE = (2(18) + 2 - 36)/2 = 1, which is the carboxylic acid's C=O only — consistent with a saturated 18:0 chain.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- fatty acid
- Long-chain carboxylic acid, e.g., CH3(CH2)16COOH
- triacylglycerol (triglyceride)
- Glycerol triester of three fatty acids
- saturated / unsaturated
- No C=C bonds / one or more C=C bonds
- cis vs trans double bond
- Same-side vs opposite-side substituents on C=C
- essential fatty acid
- Fatty acid the body cannot synthesize
- wax
- Ester of a long-chain fatty acid and a long-chain alcohol
- hydrogenation
- Addition of H2 across C=C with a metal catalyst
- rancidity
- Spoilage by oxidation or hydrolysis of fats
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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